Very high vitamin D levels in the blood exceeding 375 nmol/L or 150 ng/mL are toxic, potentially causing side effects such as loss of appetite, nausea, vomiting, muscle weakness, confusion, dehydration, excessive thirst, and kidney stones.[1] Extremely high blood levels of vitamin D can even be fatal.[2]
Since vitamin D production in the skin is self-limiting, sun exposure is unlikely to produce vitamin D levels that cause harm (although it may be possible for people with constant, high-level sun exposure, e.g., lifeguards, to synthesize harmfully high levels of Vitamin D).[3] However, vitamin D levels in the blood can readily reach toxic levels with chronic, high-level supplementation.
Although ultra-high-level supplementation is universally considered to be toxic, there’s some debate on what constitutes the upper limit for safe, “moderate-level” supplementation. Taking around 4,000 IU (100 μg) of vitamin D per day for extended periods (≥6 months) seems to increase the risk of hypercalcemia (high blood calcium) and, among older adults, the likelihood of experiencing a fall.[4]
A few trials on older adults have found that vitamin D increased the risk of falls,[5][6] and one study observed a decrease in bone mineral density among women taking high doses of vitamin D.[7]